Most subsurface openings which are made in weak underground formations are subject to eventual failure if they are not provided with some artificial means of support. Vertical and horizontal stresses in the subsurface material tend to concentrate around openings, buckling and sloughing the boundary media around the openings and causing the opening to fail.
It is common practice to prevent such failure of the mine by mechanical means; i.e., anchor bolting to maintain the integrity of the opening boundary and/or lining of the boundary with concrete and steel to counteract the closure of the opening. These methods often involve considerable expense in labor and material, and are a significant cost factor in many situations.
In underground mining, the room-and-pillar method is widely used to provide support for the overlying strata while the intermediate material is excavated. This method also has inherent limitations and cost factors, due in part to the fact that the pillars may contain some valuable ore which cannot be recovered. Yet, an increase in the ore extraction causes rapid deterioration of the opening boundary and a mounting cost of roof support, making the mining economically unfeasible. This situation has been substantially improved by the recent invention of the time-controlled multiple room mining method. Even with this method, outside rooms of the multiple room entry are expected to fail and, therefore, cannot be utilized as a safe mine opening without costly roof support work.
In non-mining excavation of large underground openings in incompetent ground formations, extensive anchor bolting is used for stabilizing the boundary of the opening. Effectiveness of this anchor bolting diminishes with weakness of the ground media in relation to the magnitude of the existing earth pressure. Therefore, formation of a stable opening in weak underground formations requires a large amount of work to reinforce the opening boundary.